With the rule astigmatism against the rule astigmatism.

What is the rule here?

The rule is in cornea vertical meridian is steeper or has more converging power than horizontal meridian.

So if any patient has steeper vertical meridian than horizontal meridian, he or she has with the rule astigmatism.

And if any patient has steeper horizontal meridian than vertical meridian, then it's called against the rule astigmatism.

Now you might be thinking why does vertical meridian is steeper than horizontal meridian in human eyes.

There are two main reasons.

One, vertical corneal diameter is shorter than horizontal corneal diameter.

And two, eyelid pressure on cornea.

Vertical meridian is normally 0.25 diameter steeper than the horizontal meridian due to pressure of eyelids.

Let's understand with the rule and against the rule astigmatism with example.

With the rule astigmatism, minus 2, minus 1, 170 degree axis.

Against the rule astigmatism, minus 3, minus 2, 100 degree axis.

Let's see how minus 2, minus 1, 170 degree axis is with the rule astigmatism.

Now let's distribute this power into the optical cross.

This is optical cross, 170 degree will be somewhere here and 80 degree will be somewhere here.

Now here minus 2 is spherical power.

So it will be in both meridian, here in 170 degree also and in 80 degree also.

And minus 1 cylinder is 170 degree axis.

So the power will be 90 degree apart or here in 80 degree.

In 80 degree meridian, we have total minus 2, minus 1 or minus 3 diameter.

And in 170 degree meridian, we have minus 2 diameter.

So to make the eye emetropic or zero power, we need to subtract 3 diameter from vertical meridian and 2 diameter from horizontal meridian.

That means vertical meridian has more converging power or steeper than the horizontal meridian.

As we already know that if vertical meridian has more converging power than horizontal meridian, then it's called with the rule astigmatism.

So minus 2, minus 1 at 170 degree axis is a with the rule astigmatism.

Now, vertical meridian doesn't mean only 90 degree and horizontal meridian doesn't mean only 180 degree.

Meridian in between 70 degree to 110 degree that is 90 degree plus minus 20 is considered vertical meridian.

And meridian in between 160 to 20 degree that is 180 degree plus minus 20 is considered horizontal meridian.

Now, tips to identify with the rule and against the rule astigmatism.

We know that power in ophthalmic lenses are present in 90 degree apart from the cylinder axis.

So minor cylinder power with axis between 160 to 20 degree that is 180 degree plus minus 20 will be with the rule astigmatism.

Because minor cylinder in 180 degree means we are subtracting power from 90 degree meridian.

So minus power in 160, 165, 170, 175, 180, 5, 10, 15 or 20 degree will be example of with the rule astigmatism.

Minor cylinder power with axis in between 70 to 110 degree that is 90 degree plus minus 20 will be against the rule astigmatism.

Because minor cylinder axis in 90 degree means we are subtracting power from 180 degree meridian that is against the rule astigmatism.

So minor cylinder in 70 degree, 75 degree, 80 degree, 85 degree, 90 degree, 95 degree, 100 degree, 105 degree, 110 degree will be example of against the rule astigmatism.

If still you have confusion regarding optical cross power and prescription power then you can watch my video about optical cross.

Link is available in the description.

Now summary of this video.

The rule is vertical meridian is steeper than horizontal meridian.

Vertical meridian is steeper because of shorter vertical corneal diameter and eyelid pressure.

Vertical meridian is generally 0.25 diameter steeper than the horizontal meridian.

Meridian in between 70 to 100 degree is considered vertical meridian.

Meridian in between 160 to 20 degree is considered horizontal meridian.

Minor cylinder power with axis in between 160 to 20 degree will be the with the rule astigmatism and minor cylinder power with axis in between 70 degree to 110 degree will be against the rule astigmatism.

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